1. Field of the Invention
The present invention relates to an OR circuit which produce an output voltage as an ORed voltage of plural input voltages.
2. Description of the Related Art
A diode-OR circuit has been broadly employed in not only digital circuits but also other electric circuits. For example, in a backup power supply circuit using a diode-OR circuit, cathodes of two diodes are connected in common to the output terminal, and main and backup power voltages are applied to respective anodes of the diodes. If the main power supply voltage is lowered for some cause, power is supplied from the backup power supply, resulting in a stable output voltage appearing on the common cathode side.
When a diode-OR circuit is used for connecting the load to the main and backup power supplies, a voltage drop inevitably occurs by a forward junction drop Vf (about 0.4 V to 0.8 V) depending on its diode type. Such a voltage drop is not ignorable in the case of a low-voltage power supply having a power voltage of 2.5 V or 3.3 V. Moreover, since the forward junction drop Vf varies depending on a current flowing through the diode, the diode-OR circuit also has a disadvantage that the output voltage varies depending on fluctuations of its load.
To overcome the above-described disadvantages, a backup power switch has been proposed in U.S. Pat. No. 4,788,450. This backup power switch uses two field-effect transistors (hereinafter abbreviated as FETs), each of which has an inherent diode as a so-called parasitic diode therein. In the backup power switch, a main power supply and a backup power supply (typically a battery) are connected to a load through respective ones of the two FETs, which are hereinafter called main-side FET and backup-side FET, respectively. The respective FETs are connected in such a way that the anode of each inherent diode is disposed on a corresponding power supply side and the cathode thereof is disposed on a load side. A control switch controls ON/OFF states of these FETs depending on the main power supply voltage. As is known well, an FET has little conduction resistance. Therefore, when the FET is brought to a conduction state, which causes its inherent diode to be short-circuited and equivalently removed from the circuit, the corresponding power supply voltage can appear on its output terminal as it is without any substantial voltage drop.
More specifically, the control switch monitors the main power supply voltage and, when the main power supply voltage falls into a predetermined proper range, sets the main-side FET to a conduction state and the backup-side FET to a non-conduction state (i.e., an inherent diode operating state). When the main power supply voltage becomes lower than the predetermined range, the control changes the main-side FET into non-conduction and the backup-side FET into conduction. Accordingly, even in the event of a main power failure, the power can be continuously supplied to the load from the backup power supply.
However, when detecting a drop of the main power supply voltage, the above-described backup power switch changes a power source from the main power supply to the backup power supply. This causes the following disadvantages.
1) The control switch monitors only the main power supply voltage and concurrently switches the respective main-side and backup-side FETs between conduction and non-conduction states depending on whether a drop of the main power supply voltage occurs. Therefore, there are cases where the voltage applied to the load is dropped instantaneously by some causes such as a time lag of the switching. In the case of an IC logical circuit, such an instantaneous voltage drop will cause an erroneous operation. Further, in the case of a reset IC incorporated therein, there is a possibility that the circuit is undesirably restarted.
2) The backup-side FET is normally in the non-conduction state where its inherent diode is in a reverse-biased state or in non-biased state and therefore the backup power supply operates unloaded without current flowing. When the main power supply voltage drops, the backup-side FET is brought into conduction, sharply increasing in load. To solve this problem, a switching power supply having a feedback circuit for keeping its output voltage constant is used as the backup power supply. In this case, however, since the feedback circuit of the switching power supply cannot follow up a rapid load fluctuation, the backup power voltage also sharply drops, inducing an erroneous operation.
An object of the present invention is to provide an inventive OR circuit allowing one stable output voltage to be produced from a plurality of input voltages.
Another object of the present invention is to provide a reliable power supply circuit which can supply a load with one stable power voltage produced from a plurality of power voltages.
According to the present invention, there is provided an OR circuit having a plurality of input terminals and a single output terminal, wherein a plurality of input voltages are applied to respective ones of the input terminals and an output voltage appears on the single output terminal. The OR circuit includes: a plurality of field-effect transistors provided for respective ones of the input terminals, each of the field-effect transistors connecting a corresponding input terminal to the single output terminal, wherein one major electrode of the field-effect transistor corresponding to an anode of an inherent diode of the field-effect transistor is connected to the corresponding input terminal and another major electrode of the field-effect transistor corresponding to a cathode of the inherent diode is connected to the single output terminal; and a plurality of controllers provided for respective ones of the field-effect transistors, wherein each of the controllers brings a corresponding field-effect transistor into a selected one of conduction state and non-conduction state depending on which one of a corresponding input voltage and the output voltage is higher than the other.
Preferably, each of the controllers brings a corresponding field-effect transistor into conduction when the output voltage is equal to or lower than a corresponding input voltage, and brings the corresponding field-effect transistor into non-conduction when the output voltage is higher than a corresponding input voltage.
According to an embodiment of the present invention, there is provided a power supply circuit using a plurality of power supplies to generate a single output voltage which is supplied to a load. The power supply circuit includes: a plurality of p-channel field-effect transistors provided for respective ones of the power supplies, each of the p-channel field-effect transistors connecting a corresponding power supply to the load, wherein a drain electrode of the p-channel field-effect transistor corresponding to an anode of an inherent diode of the p-channel field-effect transistor is connected to the corresponding power supply and a source electrode of the p-channel field-effect transistor corresponding to a cathode of the inherent diode is connected to the load; and a plurality of voltage comparators provided for respective ones of the power supplies, wherein each of the voltage comparators brings a corresponding p-channel field-effect transistor into a selected one of conduction state and non-conduction state depending on which one of a corresponding power supply voltage and the single output voltage is higher than the other.
According to another embodiment of the present invention, there is provided a power supply circuit using a plurality of power supplies to generate a single output voltage which is supplied to a load. The power supply circuit includes: a plurality of n-channel field-effect transistors provided for respective ones of the power supplies, each of the n-channel field-effect transistors connecting a corresponding power supply to the load, wherein a source electrode of the n-channel field-effect transistor corresponding to an anode of an inherent diode of the n-channel field-effect transistor is connected to the corresponding power supply and a drain electrode of the n-channel field-effect transistor corresponding to a cathode of the inherent diode is connected to the load; and a plurality of voltage comparators provided for respective ones of the power supplies, wherein each of the voltage comparators brings a corresponding n-channel field-effect transistor into a selected one of conduction state and non-conduction state depending on which one of a corresponding power supply voltage and the single output voltage is higher than the other.
According to another aspect of the present invention, there is provided a control method of an OR circuit having a plurality of input terminals and a single output terminal, wherein a plurality of input voltages are applied to respective ones of the input terminals and an output voltage appears on the single output terminal, the OR circuit comprising a plurality of field-effect transistors provided for respective ones of the input terminals, each of the field-effect transistors connecting a corresponding input terminal to the single output terminal, wherein one major electrode of the field-effect transistor corresponding to an anode of an inherent diode of the field-effect transistor is connected to the corresponding input terminal and another major electrode of the field-effect transistor corresponding to a cathode of the inherent diode is connected to the single output terminal. The method includes the steps of: a) comparing each of the input voltages with the output voltage; and b) setting a field-effect transistor corresponding to the input voltage to a selected one of conduction state and non-conduction state depending on a result of the comparing step (a).
Preferably, the step (b) includes steps of: setting the corresponding field-effect transistor to the conduction state when the output voltage is equal to or lower than the corresponding input voltage; and setting the corresponding field-effect transistor to the non-conduction state when the output voltage is higher than the input voltage.
As described above, according to the present invention, in each of a plurality of field-effect transistors, a corresponding input voltage is compared with the output voltage, and the conduction/non-conduction state of a corresponding field-effect transistor is controlled depending on a corresponding comparison result. Therefore, the conduction/non-conduction states of the respective field-effect transistors are individually controlled and all the field-effect transistors are not uniformly controlled. This can effectively avoid a sharp change of the output voltage. As a result, the output voltage can be kept in a remarkably stable state.